Well it arrived today and so I just thought I would give my initial impressions and perhaps do a more comprehensive review at a later date. This is the Nikon version and used on a D300.
Build & size
Below are some images comparing it to its bigger brother, the Tamron 90 f2.8. Obviously the 60 is more compact though only slightly lighter than the 90 (390g vs 405g). Both have a 55mm filter and thickness is about the same. The second image shows the 90mm fully extended @ the maximum magnification whereas the 60 is completely internal focusing which is one reason why wanted this version.
The AF on the 60mm is very good and quite snappy for a macro. Unfortunately the AF on my 90 is not working at the moment and I only discovered this today when I wanted to do a comparison. However I would have to say the AF on the 60 would be significantly faster and much quieter than the 90. Seems to focus ok in low light as well which is great for portrait work.
The AF ring is a bit stiff on the 60 but I'm sure it will loosen up with use. The 60 has a small AF/MF switch on the left side in contrast to the clutch arrangement on the focus ring of the 90. Below are some quick grab shots just using AF and the centre point on the D300 (it was a bit windy so would have liked faster shutter speeds for optimum sharpness). These are natural light and handheld.
Haven't done any head to head tests against the 90 yet but judging by the images I've taken so far, sharpness seems about the same and CA is very well controlled and I suspect better than the 90.
For those interested in using this lens for portraits I have included a few images below shot at f2.2 (the lens aperture only opens to f2 at near infinity so you won't be able to use it 'wide open' for normal portrait shots). It stays at f2.2 within about 0.6m of the subject and then jumps up to 2.4 (those familiar with macro lenses will understand that the effective aperture gets smaller as you near the minimum focusing distance).
Some bokeh tests @ f2.2
A couple of portraits (Matthew my eldest son) and 100% crops @ f2.2. These are SOOC with in-camera sharpening set to +5 which is moderate.
While it's not exactly tack sharp at f2.2 it's pretty decent if you ask me and certainly more than useable.
Working distance and magnification
Working distance at 1:1 is about the same as the 90mm (100mm). However with the 68mm Kenko tubes it actually has more WD (75mm) from the front filter than the 90 (65mm). This is a pleasant surprise and for me at least, that extra 10mm will be very welcome. Magnification is also higher with the tubes topping out at 2.36x versus 2.0x on the 90. This means that according to the formula my 60mm macro is actually 50mm at its minimum focus distance.
Overall for me this will be a worthwhile upgrade with the extra WD, internal focusing, and higher magnification with tubes. The faster AF and good performance at f2.2 are just icing on the cake The only possible issue is that my home made ring flash adapter will have to fit over (and cover the front part of) the focus ring, but it shouldn't be too much of a problem.